1. Field of the Invention
The present invention relates generally to a reflow soldering, and more particularly, to a technique for performing soldering by use of a heater tip.
2. Description of the Related Art
The reflow soldering is a technique for effecting a metal-to-metal joining by melting a solder through heating, the solder being interposed between soldered sites of metal members to be joined together. The reflow soldering has hitherto widely used in fabrication, mounting, etc., of electronic components.
A conventional reflow soldering apparatus using a heater tip as its heating means is provided with a single-phase AC power supply as shown in FIG. 7. In this power supply, a single-phase AC voltage V of a commercial frequency applied to input terminals 100 and 102 are fed via a contactor consisting of a pair of thyristors 104 and 106 to a primary coil of a step-down transformer 108. An AC induced electromotive force (secondary voltage) occurring at the secondary coil of the transformer 108 is applied via a secondary conductor to terminals 114a and 114b of a heater tip 114, so that a secondary current i2 having a larger current value than that of a primary current i1 flows as a supplied current I for heating heater tip through the secondary circuit.
When supplied with the current, the heater tip 114 generates heat as a result of resistance heating, and via its point, applies pressure and heat to the sites to be soldered of the workpiece 116. Cream solder is applied in advance onto joint areas of the sites to be soldered and melts under the action of the pressure and heat from the heater tip 114. Upon the elapse of a given time the current supply is ceased and then the pressure is also released. As a result, the solder solidifies to physically and electrically join the sites to be soldered of the workpiece 116 together by way of the solidified solder.
The magnitude (effective value) of the supplied current I is determined by the conduction angle. However, a substantially constant relationship lies between the firing angle and the conduction angle, and hence it may be said that the magnitude of the supplied current I is determined by the firing angle. In this power supply, the temperature at the point of the heater tip 114 is detected by a temperature sensor 118 comprised of a thermocouple for example. In order to allow the detected temperature (heater tip temperature) to be coincident with a set temperature, the control unit 110 determines a firing angle xcex8 at every half cycle (see FIG. 8) so that the thyristors 104 and 106 are fired (turned on) at the firing angle xcex8 by way of a firing circuit 112.
The conventional reflow soldering apparatus using such a single-phase AC power supply has employed the current-supplying control in which the temperature feedback is performed at a considerably long cycle time TH in the field of electronic control, as much as the half cycle (10 ms or 8.33 ms) of the commercial frequency (50 Hz or 0 Hz), which has made it difficult to cause the heater tip temperature to rapidly and precisely coincide with the set temperature. As is apparent from FIG. 8, the supplied current I has a distorted waveform derived from the phase control of a sinusoidal wave and further has a quiescent time at every half cycle of the commercial frequency, so that the heater tip temperature may undergo a large ripple and especially upon the rise, a stepwise temperature rise may appear as shown in FIG. 9, resulting in unstable temperature characteristics.
Furthermore, workpieces like microelectronic components are recently prevailing which require metal-to-metal bonding presenting a high reliability in a brief process time less than several tens of milliseconds. It would be difficult for the conventional reflow soldering apparatus to deal with such workpieces.
The present invention was conceived in view of the above problems. It is therefore an object of the present invention to provide a reflow soldering apparatus having an improved heater tip temperature control.
Another object of the present invention is to provide a reflow soldering apparatus capable of rapidly and precisely controlling the temperature of the heater tip.
A further object of the present invention is to provide a reflow soldering apparatus capable of effecting a high reliable soldering regardless of a short process time.
In order to attain the above objects, according to an aspect of the present invention there is provided a reflow soldering apparatus comprising a heater tip for generating heat by current supply; pressing means for pressing a point of the heater tip against sites to be soldered, for pressing over a predetermined pressing time; a rectifying circuit for rectifying an AC voltage of a commercial frequency into a DC voltage; an inverter for converting the DC voltage output from the rectifying circuit into a pulsed voltage of a high frequency; a transformer having primary side terminals electrically connected to output terminals of the inverter, the transformer having secondary side terminals electrically connected to terminals of the heater tip without intervention of any rectifying circuit; and inverter control means for segmenting a current-supplying time defined in the predetermined pressing time into a plurality of current-supplying periods, the inverter control means providing a control of switching operations of the inverter such that in odd-numbered current-supplying periods the inverter issues the high-frequency pulse with one polarity and such that in even-numbered current-supplying periods the inverter issues the high-frequency pulse with the other polarity.
In the present invention, the heater tip accepts a supplied current whose waveform has been controlled at a high-frequency cycle by the inverter, with the result that a higher heat-generating efficiency of the heater tip is ensured so that the solder can rapidly melt in a brief period of time to securely achieve a desired soldering.
To achieve a more precise and stable temperature control in the reflow soldering apparatus of the present invention, preferably the apparatus further comprises temperature detecting means for detecting the temperature of the vicinity of the point of the heater tip, and the inverter control means provide a control of switching operations of the inverter such that a detected temperature from the temperature detecting means is fedback at every any desired cycles of the high frequency so as to allow the detected temperature to coincide with a set temperature.
To achieve the above more precise and stable temperature control and to achieve a more rapid and stable temperature rise of the heater tip in the reflow soldering apparatus of the present invention, preferably, the apparatus further comprises temperature detecting means for detecting the temperature of the vicinity of the point of the heater tip; and current measuring means for measuring the current on the primary side or secondary side of the transformer. The inverter control means may provide a control of switching operations of the inverter such that a measured current from the current measuring means is fedback immediately after the start of current supply in the current-supplying time so as to allow the measured current to coincide with a set current, the inverter control means monitoring a detected temperature obtained from the temperature detecting means to provide a control of switching operations of the inverter such that a detected temperature from the temperature detecting means is fedback after the detected temperature reaches a set temperature or a predetermined temperature in the vicinity of the set temperature so as to allow the detected temperature to coincide with the set temperature.